Decomposition of refuse within a landfill produces landfill gas which contains methane. In some instances, the methane concentration is sufficient to warrant recovery of the landfill gas so that the methane can be used as a fuel. To increase the concentration of the methane, a methane purification plant can be installed at the landfill.
The recovery of landfill gas and its purification requires a very substantial investment for the construction of the necessary recovery and purification plant. Before making an investment of this magnitude, it is imperative to make numerous determinations regarding properties of the landfill gas, including, among other things, the composition of the landfill gas, the total volume of the landfill gas which the landfill can produce and whether or not the rate of landfill gas generation by the landfill is sufficient to warrant the installation of the recovery and purification plant. The landfill gas generation rate is the rate at which the landfill or a quantity of refuse produces landfill gas, and it may be measured, for example, in cubic feet per minute per given volume or weight of refuse.
Also, the rate of landfill gas generation must be known so that the allowable withdrawal rate of the landfill gas from the landfill can be ascertained. If the withdrawal rate of the landfill gas in the landfill exceeds the landfill gas generation rate, air will be drawn into the landfill through the surface of the landfill. An entry into the landfill is totally intolerable because the oxygen poisons the micro-organisms which are essential to the production of methane by the landfill. Accordingly, air entry into the landfill as a result of overestimating the rate of gas generation must be completely avoided.
Landfills may extend over many acres and contain huge volumes of refuse. For this reason, it is impossible to directly measure the rate of landfill gas generation of the entire landfill.
One prior art attempt at determining the instantaneous rate of gas generation from refuse is to place new refuse in a lysimeter, drip water through the lysimeter and measure the gas generated. Although experiments of this type are of interest in a laboratory, they have been found to be essentially useless for determining the rate of landfill gas generation by a landfill.
Another prior art method is disclosed in Johnson et al U.S. Pat. No. 4,026,355. Although this method has been used satisfactorily, it requires the installation of a multiplicity of wells and probes in the landfill.
Another disadvantage of these prior art methods is that they do not provide any way to determine the effect of certain variables on properties of the landfill gas. For example, it is sometimes necessary or desirable to determine whether the addition of a different kind of refuse not previously deposited in the landfill will adversely affect the landfill gas generation rate. Conversely, it may be desirable to determine whether a particular material may act as a stimulant to increase the gas generation rate. It is, of course, not feasible to expose the entire landfill to the new ingredient because, if it adversely affects the rate of gas generation or gas composition, a substantial fuel loss from the landfill will occur. Other variables which must also be considered include temperature and moisture flow within the landfill.